The polyphenylene ethers are known and described in numerous publications including Hay, U.S. Pat. Nos. 3,306,874 and 3,306,875 and Stamatoff U.S. Pat. Nos. 3,257,357 and 3,257,358. The high molecular weight polyphenylene ethers are high performance engineering thermoplastics possessing relatively high melt viscosities and softening points -- i.e., in excess of 275.degree. C., and are useful for many commercial applications requiring high temperature resistance including formation of films, fibers and molded articles.
Although they have the above-described desirable properties, it is also known that certain properties of the polyphenylene ether resins are undesirable for some commercial uses. For example, parts molded from the polyphenylene ethers are somewhat brittle due to poor impact strength. In addition, the relatively high melt viscosities and softening points are considered a disadvantage for many uses. Films and fibers can be formed from polyphenylene ether resins on a commercial scale using solution techniques, but melt processing is commercially unattractive because of the high temperatures required to soften the resin and the problems associated therewith such as instability, discoloration and the requirement for specially designed process equipment to operate at elevated temperatures. Molded articles can be formed by melt processing techniques, but, again, the high temperatures required are undesirable.
It is known in the art that properties of the polyphenylene ether resins can be materially altered by blending them with other resins. For example, one method for improving the melt processability of the polyphenylene ethers is disclosed in a commonly-assigned patent, U.S. Pat. No. 3,379,792, incorporated herein by reference. According to this patent, flow properties of the polyphenylene ethers are improved by blending with from about 0.1 to 25 parts by weight of a polyamide. In another commonly-assigned patent, U.S. Pat. No. 3,361,851, a polyphenylene ether composition comprising a polyphenylene ether blended with a polyolefin is disclosed. The polyolefin is added to improve impact strength and resistance to aggressive solvents. In a third commonly-assigned patent, Cizek, U.S. Pat. No. 3,383,435, there are provided means for simultaneously improving the melt processability of the polyphenylene ether resins while simultaneously up-grading many properties of polystyrene homopolymer and random copolymer resins. The invention of the Cizek patent is based upon the discovery that the polyphenylene ether resins and such polystyrene resins, including rubber modified polystyrene resins, are combinable in all proportions and result in compositions having many properties improved over those of either of the components.
One preferred embodiment of the Cizek patent is a composition comprising a high-impact, rubber reinforced polystyrene and a poly(2,6-dialkyl-1,4-phenylene)ether. This composition was preferred because it provides the aforementioned objectives of improving the melt-processability properties of the polyphenylene ether resin and provides the further advantage of improving impact resistance of parts molded from the blend. Furthermore, the Cizek composition of the polyphenylene ether and the high impact polystyrene could be custom-formulated to provide predetermined properties ranging between those of the polystyrene and those of the polyphenylene ether by controlling the ratio of the two polymers. The reason for this is that the blend exhibits a single set of thermodynamic properties rather than two distinct sets of properties -- i.e., one for each of the components of the blend as is typical with blends of prior art.
The styrene resins disclosed in the Cizek patent are either homopolymers or random copolymers. For example, the crystal polystyrenes of Examples 1 and 9 are homopolymers. Lutrex HT-88 of Example 7 is a commercial styrene grafted butadiene rubber modified high impact polystyrene. In such products a portion of the styrene is homopolymerized into side chains onto a rubber backbone. The styrene containing copolymer resins disclosed in Cizek, Col. 3, are random copolymers: styrene acrylonitrile, styrene-butadiene, styrene-acrylonitrile- .alpha.-alkyl styrene copolymers, styrene-acrylonitrile-butadiene (ABS), copolymers of ethylvinyl benzene and divinyl benzene and the like. With the exception of styrene-acrylonitrile- .alpha.-methyl styrene, Example 17, none of the Cizek terminology can be construed to disclose a block copolymer of the A-B-A' 1 type. Because the monomers are grafted into terminal blocks, instead of side chains, A-B-A' block copolymers are more linear and their properties differ markedly from the grafted rubber copolymers used in Cizek. Moreover, because there is no disclosure of any copolymer with an elastomeric center block, the Cizek compositions do not include combinations of polyphenylene ethers with elastomeric block copolymers of vinyl aromatic compounds and conjugated dienes.
With respect to the preferred embodiments in the Cizek patent, it is believed that the impact resistance of the polyphenylene ether is improved because of the diene rubber content in the random grafted high-impact polystyrene and in the ABS resin and, in this respect, the improvement in impact strength appears to be directly proportional to the diene rubber content of the polystyrene resin or the ABS resin, increasing concentrations of diene rubber resulting in increased impact strength. However, it has also been found -- as a disadvantage -- that the gloss of parts molded from the polyphenylene ether resin and the high-impact polystyrene resin is inversely proportional to the diene rubber content and that, therefore, as the diene rubber content is increased, gloss and surface appearance of the molded parts are decreased. Consequently, increasing the diene rubber content of the compositions results in increased impact strength, but with a sacrifice in surface appearance and gloss. Alternatively, reduction in diene rubber content such as by the use of unreinforced (crystal) polystyrene results in parts having good gloss, but at a sacrifice in impact strengths. Because both impact strength and gloss are commercially important properties in the manufacture of molded parts, although the preferred compositions of the Cizek patent provide the advantages noted above, it has been found difficult to provide compositions having both optimum impact strength and surface appearance.
Also, as mentioned above, the polyphenylene ether-styrene resin compositions of the Cizek patent were disclosed only to be improved in resistance to aggressive organic solvents, by copolymerization of the styrene resin with an alkenyl cyanide compound, e.g. acrylonitrile (Examples 10-12). Therefore, the need still exists for means to more easily provide compositions with outstanding resistance to gasoline.
The Lauchlan patent, U.S. Pat. No. 3,660,531, discloses blends of polyphenylene ether resins, elastomeric block copolymers and polystyrene resins. The compositions taught by Lauchlan have a polyphenylene ether-alkenyl aromatic resin matrix that constitutes greater than 50% of the blend, with a proviso that the ratio of polyphenylene ether resin to alkenyl aromatic resin is always greater than 1. Applicants have discovered that the compositions, wherein the ratio of polyphenylene ether resin to alkenyl aromatic resin is less than 1, have a lower melt viscosity and are more stable than the Lauchlan type compositions. The compositions of the present invention have good impact strengths which make then useful for many molding applications. In addition the compositions which contain at least about 10% by weight of the elastomeric block copolymer are resistant to attack by aggressive solvents such as gasoline. These compositions may also be reinforced with fibrous glass with enhancement in properties and no loss of excellent resistance to gasoline environments. All such compositions have good impact strengths and good resistance to distortion by heat.